JP4184860B2 - Stainless steel and structures - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、原子力プラント等の高温高圧水環境下において高耐食高強度を要求される部材に用いられる耐熱時効劣化特性に優れたステンレス鋼および当該ステンレス鋼によって構成された構造物に関する。
【０００２】
【従来の技術】
従来の原子力プラントにおいては、各種弁、弁棒、ボルトおよび制御棒駆動系機器等の高耐食及び、高強度を要求される部分に析出硬化型ステンレス鋼あるいはマルテンサイト系ステンレス鋼が使用されている（特許文献１参照）。
【０００３】
【特許文献１】
特開平６−３４６１９８号公報
【０００４】
【発明が解決しようとする課題】
しかしながら、析出硬化型ステンレス鋼およびマルテンサイト系ステンレス鋼は、高温に長時間加熱されるとスピノーダル分解を起こして靱性及び延性の低下を生じることが研究結果からわかった。スピノーダル分解は、組織の中のマルテンサイト相およびフェライト相が長時間加熱されることによりＦｅ原子とＣｒ原子が周期的に分離する相分離、つまり、Ｆｅリッチ相とＣｒリッチ相に分離する現象である。
【０００５】
本発明の目的は、高温高圧水環境で長期間使用した場合においても靱性及び延性の低下の少ないステンレス鋼および当該ステンレス鋼によって構成された構造物を提供することを目的とする。
【０００６】
【課題を解決するための手段】
上記目的を達成するため、請求項１の発明のステンレス鋼は、重量％でＣｒを１２％〜２６％、Ｎｉを４％〜１６％、Ｃを０．１０％以下、Ｓを０．０１％以下、Ｓｉを１．００％以下、Ｍｎを２．００％以下、Ｍｏを０．１〜４％、Ｃｕを２〜５％、Ｎｂを０．５〜１．５％含有し、残部がＦｅおよび不可避的不純物からなり、４５０〜６５０℃の熱処理によりＣｕが析出され、５９３℃〜９２７℃の熱処理により金属間化合物Ｎｉ ３ Ｎｂが析出されている構成とする。
【０００７】
Ｃｒが１２％以上になると表面に安定した酸化被膜が形成され、耐食性が非常に良好になる。従って、Ｃｒ１２％以上と限定する。また、ステンレス鋼の組織を成分により分類したシェフラー線図上でＣｒ１２％以上、かつマルテンサイト組織となる範囲を読み取り、Ｃｒの上限は２６％とする。また、Ｎｉの場合、シェフラー線図からマルテンサイト組織を示すのは１６％以下である。よって、Ｎｉの上限は１６％とする。
さらに、Ｃｕを２〜５％、Ｎｂを０．５〜１．５％含有する。
Ｃｕは析出硬化に寄与する元素であり、２％以下では析出硬化の寄与が小さく、５％以上では析出物が粗大化し、脆化を促進させるので２〜５重量％とする。そして、熱処理が４５０℃以下では析出元素であるＣｕが充分析出しないことと合金の靭性を得るための焼戻しが充分されない。また６５０℃以上ではＣｕ以外の析出物、例えばＣｒ炭化物が析出し、合金の耐食性に悪影響を与える可能性がある。
Ｎｂは析出硬化に寄与する元素であり、０．５％以下では析出硬化の寄与が小さく、１．５％以上では析出物が粗大化し、脆化を促進させるので０．５〜１．５％とする。また、γ '' 相であるＮｉ ３ Ｎｂを析出させるために、金属間化合物の時間−温度−析出物の関係図より、熱処理温度の下限を５９３℃、上限を９２７℃とする。
【００１５】
請求項２の発明は構造物であり、請求項１に記載のステンレス鋼によって構成され高温高圧水環境で使用される構成とする。
【００１６】
【発明の実施の形態】
以下、本発明の実施の形態について説明する。なお、本実施の形態において成分元素の含有量を示すパーセンテージは全て重量基準のパーセンテージ（ｗｔ％）を示す。
【００１７】
本発明の第１の実施の形態に係る合金は、Ｃｒを１２〜２６％含有するステンレス鋼を基本とする。そして、原子炉水環境に長期間使用した場合における靭性及び延性の低下を抑制するためにＭｏを０．１〜４％添加する。
【００１８】
ＭｏはＣｒと共存することにより導電性水中における耐孔食性の向上に寄与する元素である。その効果を発揮させるには０．１％以上の添加が必要である。しかし、その添加量が４％を超えると、材料の脆化の問題が生じるため、添加量の上限を４％とする。
【００１９】
Ｃは強度を向上させる元素であるが、耐食性に悪影響を与える元素でもある。Ｃは熱処理あるいは溶接熱の影響により粒界にＣｒ炭化物を形成し、それに伴い粒界近傍にＣｒ欠乏層を形成し耐食性を低下させる。そのため、Ｃは０．１０％以下にすることが望ましい。
【００２０】
Ｓは溶解時の脱酸剤として不可避的に混入するＭｎとともに非金属介在物ＭｎＳを生成する元素であり、その含有量の増加に伴い耐食性に悪影響を与える。そのため、Ｓ含有量は０．０１％以下とする。
【００２１】
ＣやＳを多く含むと、ＭｎＳ等の非金属介在物やＣの偏析部が生じる。これにより、合金が高温水中に曝されたとき、これらの部位とその周囲の組織の間に電位差が生じ非金属介在物およびＣ偏析部の存在する部位が選択的に腐食する。本実施の形態の合金は、このような現象を防止するためにＣおよびＳ含有量を調整する。
【００２２】
Ｎｉは４〜１６％含有することが好ましい。Ｎｉ添加により靭性および耐食性が向上する。以上述べた元素以外の成分元素（不純物元素）、例えばＳｉ、Ｍｎ、Ｐ等は通常のＪＩＳ規格（ＪＩＳ Ｇ３２１４）に規定しているレベルに設定して問題ない。
【００２３】
つぎに、本発明の第２の実施の形態を説明する。
この実施の形態に係る合金は、Ｃｒを１２〜２６％、Ｎｉを４〜１６％、Ｃを０．１０％以下、Ｓを０．０１％以下含み、熱時効に伴うＣｒリッチ相形成を抑制するためにＭｏを０．１〜４％添加し、残部がＦｅおよび不可避的不純物からなる。更に強度を向上させるために、Ｃｕを２〜５％あるいはＡｌを０．５〜１．５％添加する。あるいはＮｂを０．５〜１．５％あるいはＴｉを０．５〜１．５％添加する。
【００２４】
以下に、強度向上のため添加する元素の含有量の限定理由について説明する。Ｃｕは、４５０〜６５０℃で熱処理を行うと母材中に析出し、いわゆる析出硬化により強度が上昇する。２％未満では析出硬化の寄与が小さく、５％を越えると脆化が促進するため２〜５％の添加が望ましい。
【００２５】
Ａｌは、６２１〜８４３℃で熱処理を行うと母材中に金属間化合物Ｎｉ３Ａｌとして析出し、いわゆる析出硬化により強度が上昇する。０．５％未満では析出硬化の寄与が小さく、１．５％を越えると脆化が促進するため０．５〜１．５％の添加が望ましい。
【００２６】
Ｎｂは、５９３〜９２７℃で熱処理を行うと母材中に金属間化合物Ｎｉ３Ｎｂとして析出し、いわゆる析出硬化により強度が上昇する。０．５％未満では析出硬化の寄与が小さく、１．５％を越えると脆化が促進するため０．５〜１．５％の添加が望ましい。
【００２７】
Ｔｉは、６２１〜８４３℃で熱処理を行うと母材中に金属間化合物Ｎｉ３Ｔｉとして析出し、いわゆる析出硬化により強度が上昇する。０．５％未満では析出硬化の寄与が小さく、１．５％を越えると脆化が促進するため０．５〜１．５％の添加が望ましい。
【００２８】
つぎに、本発明の第３の実施の形態に係る合金は、Ｃｒを１２〜１４％、Ｎｉを６〜１０％、Ｃを０．１０％以下、Ｓを０．０１％以下含み、残部がＦｅおよび不可避的不純物からなる。
【００２９】
以下、各成分元素の含有量の限定理由について説明する。
本実施の形態においてＣｒを１２〜１４％、Ｎｉを６〜１０％としたのは、金属組織をフェライト相のないマルテンサイト組織単相にするために決定したものである。フェライト相は、マルテンサイト相に比べＣｒ含有量が高く、スピノーダル分解が生じ易く経年劣化を促進するＣｒリッチ相を形成しやすいためである。
【００３０】
つぎに、本発明の第４の実施の形態に係る合金は、Ｃｒを１２〜１８％、Ｎｉを８〜１０％、Ｃを０．１０％以下、Ｓを０．０１％以下含み、残部がＦｅおよび不可避的不純物からなる。
【００３１】
以下、各成分元素の含有量の限定理由について説明する。
Ｃｒを１２〜１８％、Ｎｉを８〜１０％としたのは、金属組織をフェライト相とオーステナイトとの混相組織にするために決定したものである。フェライト相は、マルテンサイト相に比べＣｒ含有量が高く、スピノーダル分解が生じ易く経年劣化を促進するＣｒリッチ相を形成しやすいためである。
【００３２】
次に、図１，図２を参照して本発明の実施の形態の合金についてのデータを比較例と比較しつつ説明する。図１は合金の組成を示し、図２は４００℃における時効に伴う衝撃値変化率を示す。
【００３３】
これらの図において、開発材Ａは上記第１の実施の形態の合金であり、開発材Ｂは上記第２の実施の形態の合金であり、開発材Ｃは上記第３の実施の形態および上記第４の実施の形態に対応する組成の合金である。また、従来材ＤはＳＵＳ４３１ステンレス鋼であり、従来材ＥはＳＵＳ６３０ステンレス鋼である。
【００３４】
開発材Ａに対して１０２０℃×１時間＋６４０℃×１時間の熱処理を施し、従来材Ｄに対して１０５０℃×１時間＋７００℃×８時間の熱処理を施したのち４００℃で時効を行ない、シャルピー衝撃試験を実施した。その結果、開発材Ａは、衝撃値変化率の低下が従来材Ｄに比べ長時間側となっており、経年劣化が抑制されているといえる。
【００３５】
開発材Ｂと従来材ＥはともにＣｕを含有しているが、開発材Ｂに対して１０３８℃×１時間＋５９３℃×４時間の熱処理を、従来材Ｅに対して１０５０℃×１時間＋５８０℃×４時間の熱処理を施したのち４００℃で時効を行ない、シャルピー衝撃試験を実施した。その結果を比較すると、開発材Ｂは衝撃値変化率の低下が従来材Ｅに比べ長時間側となっており、経年劣化が抑制されているといえる。
【００３６】
次に、開発材Ｃと従来材Ｄ及び従来材Ｅはともにフェライト相を有するが、開発材Ｃに９２７℃×１時間＋５９３℃×４時間の熱処理を施したのち４００℃で時効を行ない、シャルピー衝撃試験を実施した結果で比較すると、開発材Ｃは、衝撃値変化率の低下が従来材Ｄ及び従来材Ｅに比べ長時間側となっており、経年劣化が抑制されているといえる。
【００３７】
【発明の効果】
本発明によれば、高温高圧水環境で長期間使用した場合においても靱性及び延性の低下の少ないステンレス鋼および当該ステンレス鋼によって構成された構造物を提供することができる。
【図面の簡単な説明】
【図１】本発明の実施の形態による開発材と従来材の組成を示す表。
【図２】図１の表に示した開発材及び従来材の４００℃時効による衝撃値の変化を示すグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a stainless steel excellent in heat aging deterioration characteristics used for a member that requires high corrosion resistance and high strength in a high-temperature and high-pressure water environment such as a nuclear power plant, and a structure constituted by the stainless steel.
[0002]
[Prior art]
In conventional nuclear power plants, precipitation hardened stainless steel or martensitic stainless steel is used in parts that require high corrosion resistance and high strength, such as various valves, valve rods, bolts and control rod drive system equipment. (See Patent Document 1).
[0003]
[Patent Document 1]
JP-A-6-346198 [0004]
[Problems to be solved by the invention]
However, it has been found from the research results that precipitation hardening type stainless steel and martensitic stainless steel cause spinodal decomposition when heated to a high temperature for a long time, resulting in a decrease in toughness and ductility. Spinodal decomposition is a phase separation in which Fe and Cr atoms are periodically separated when the martensite phase and ferrite phase in the structure are heated for a long time, that is, a phenomenon in which the Fe rich phase and the Cr rich phase are separated. is there.
[0005]
An object of the present invention is to provide a stainless steel and a structure constituted by the stainless steel, which are less deteriorated in toughness and ductility even when used in a high temperature and high pressure water environment for a long period of time.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the stainless steel according to the first aspect of the present invention is 12% to 26% Cr, 4% to 16% Ni, 4% to 16% C, 0.10% or less C, 0.01% S Hereinafter, Si is 1.00% or less, Mn is 2.00% or less, Mo is 0.1 to 4% , Cu is 2 to 5%, Nb is 0.5 to 1.5%, and the balance is Fe. and Ri Do unavoidable impurities, Cu is deposited by thermal treatment of 450 to 650 ° C., 593 ° C. intermetallic compound Ni 3 Nb by heat treatment ～927 ° C. is configured that has been deposited.
[0007]
When Cr is 12% or more, a stable oxide film is formed on the surface, and the corrosion resistance becomes very good. Therefore, it is limited to Cr 12% or more. Further, on the Schaeffler diagram in which the structure of stainless steel is classified by component, the range of Cr 12% or more and the martensite structure is read, and the upper limit of Cr is 26%. In the case of Ni, the martensitic structure is 16% or less from the Schaeffler diagram. Therefore, the upper limit of Ni is 16%.
Furthermore, 2 to 5% of Cu and 0.5 to 1.5% of Nb are contained .
Cu is an element that contributes to precipitation hardening. If it is 2% or less, the contribution of precipitation hardening is small, and if it is 5% or more, the precipitate becomes coarse and promotes embrittlement. When the heat treatment is 450 ° C. or less, Cu as a precipitation element is not sufficiently precipitated and tempering for obtaining the toughness of the alloy is not sufficient. Further, at 650 ° C. or higher, precipitates other than Cu, such as Cr carbide, may be deposited, which may adversely affect the corrosion resistance of the alloy.
Nb is an element that contributes to precipitation hardening. If it is 0.5% or less, the contribution of precipitation hardening is small, and if it is 1.5% or more, the precipitate becomes coarse and promotes embrittlement. And Further, in order to precipitate Ni 3 Nb which is a γ ″ phase, the lower limit of the heat treatment temperature is set to 593 ° C. and the upper limit is set to 927 ° C. from the relationship diagram of time-temperature-precipitate of the intermetallic compound.
[0015]
The invention of claim 2 is a structure, and is constituted by the stainless steel of claim 1 and used in a high temperature and high pressure water environment.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below. In the present embodiment, all percentages indicating the content of component elements are percentages (wt%) based on weight.
[0017]
The alloy according to the first embodiment of the present invention is based on stainless steel containing 12 to 26% of Cr. And 0.1-4% of Mo is added in order to suppress the fall of toughness and ductility when it is used for a long time in the reactor water environment.
[0018]
Mo is an element that contributes to the improvement of pitting corrosion resistance in conductive water by coexisting with Cr. In order to exert the effect, addition of 0.1% or more is necessary. However, if the addition amount exceeds 4%, the problem of material embrittlement occurs, so the upper limit of the addition amount is set to 4%.
[0019]
C is an element that improves strength, but is also an element that adversely affects corrosion resistance. C forms Cr carbide at the grain boundary due to the influence of heat treatment or welding heat, and accordingly, forms a Cr-deficient layer near the grain boundary to lower the corrosion resistance. Therefore, C is desirably 0.10% or less.
[0020]
S is an element that forms non-metallic inclusions MnS together with Mn which is inevitably mixed as a deoxidizing agent at the time of dissolution, and adversely affects corrosion resistance as the content increases. Therefore, the S content is 0.01% or less.
[0021]
When a large amount of C or S is contained, nonmetallic inclusions such as MnS or segregated portions of C are generated. As a result, when the alloy is exposed to high temperature water, a potential difference is generated between these sites and the surrounding structure, and the sites where nonmetallic inclusions and C segregated portions exist are selectively corroded. In the alloy of the present embodiment, the C and S contents are adjusted in order to prevent such a phenomenon.
[0022]
It is preferable to contain 4 to 16% of Ni. Addition of Ni improves toughness and corrosion resistance. Component elements (impurity elements) other than the elements described above, such as Si, Mn, P, etc., can be set to the levels defined in the normal JIS standard (JIS G3214) without any problem.
[0023]
Next, a second embodiment of the present invention will be described.
The alloy according to this embodiment includes Cr of 12 to 26%, Ni of 4 to 16%, C of 0.10% or less, and S of 0.01% or less, and suppresses the formation of a Cr rich phase due to thermal aging. For this purpose, Mo is added in an amount of 0.1 to 4%, and the balance is composed of Fe and inevitable impurities. In order to further improve the strength, 2 to 5% of Cu or 0.5 to 1.5% of Al is added. Alternatively, 0.5 to 1.5% of Nb or 0.5 to 1.5% of Ti is added.
[0024]
The reason for limiting the content of elements to be added for improving the strength will be described below. Cu is precipitated in the base material when heat-treated at 450 to 650 ° C., and the strength is increased by so-called precipitation hardening. If it is less than 2%, the contribution of precipitation hardening is small, and if it exceeds 5%, embrittlement is promoted, so addition of 2 to 5% is desirable.
[0025]
When Al is heat-treated at 621 to 843 ° C., it precipitates as an intermetallic compound Ni 3 Al in the base material, and the strength increases by so-called precipitation hardening. If it is less than 0.5%, the contribution of precipitation hardening is small, and if it exceeds 1.5%, embrittlement is promoted, so addition of 0.5 to 1.5% is desirable.
[0026]
When heat treatment is performed at 593 to 927 ° C., Nb precipitates as an intermetallic compound Ni 3 Nb in the base material, and the strength increases by so-called precipitation hardening. If it is less than 0.5%, the contribution of precipitation hardening is small, and if it exceeds 1.5%, embrittlement is promoted, so addition of 0.5 to 1.5% is desirable.
[0027]
When Ti is heat-treated at 621 to 843 ° C., it precipitates as an intermetallic compound Ni 3 Ti in the base material, and the strength increases by so-called precipitation hardening. If it is less than 0.5%, the contribution of precipitation hardening is small, and if it exceeds 1.5%, embrittlement is promoted, so addition of 0.5 to 1.5% is desirable.
[0028]
Next, the alloy according to the third embodiment of the present invention includes 12 to 14% of Cr, 6 to 10% of Ni, 0.10% or less of C, 0.01% or less of S, and the balance being It consists of Fe and inevitable impurities.
[0029]
Hereinafter, the reason for limiting the content of each component element will be described.
In this embodiment, the reason why Cr is set to 12 to 14% and Ni is set to 6 to 10% is determined in order to make the metal structure a single martensite structure having no ferrite phase. This is because the ferrite phase has a higher Cr content than the martensite phase and is likely to cause spinodal decomposition, and easily forms a Cr-rich phase that promotes deterioration over time.
[0030]
Next, the alloy according to the fourth embodiment of the present invention includes 12 to 18% of Cr, 8 to 10% of Ni, 0.10% or less of C, 0.01% or less of S, and the balance being It consists of Fe and inevitable impurities.
[0031]
Hereinafter, the reason for limiting the content of each component element will be described.
The reason why Cr is 12 to 18% and Ni is 8 to 10% is determined in order to make the metal structure a mixed phase structure of a ferrite phase and austenite. This is because the ferrite phase has a higher Cr content than the martensite phase and is likely to cause spinodal decomposition, and easily forms a Cr-rich phase that promotes deterioration over time.
[0032]
Next, referring to FIG. 1 and FIG. 2, data on the alloy of the embodiment of the present invention will be described in comparison with a comparative example. FIG. 1 shows the composition of the alloy, and FIG. 2 shows the rate of change in impact value with aging at 400 ° C.
[0033]
In these drawings, the developed material A is the alloy of the first embodiment, the developed material B is the alloy of the second embodiment, and the developed material C is the above-described third embodiment and the above-described embodiment. It is an alloy having a composition corresponding to the fourth embodiment. Further, the conventional material D is SUS431 stainless steel, and the conventional material E is SUS630 stainless steel.
[0034]
The developed material A is subjected to a heat treatment of 1020 ° C. × 1 hour + 640 ° C. × 1 hour, the conventional material D is subjected to a heat treatment of 1050 ° C. × 1 hour + 700 ° C. × 8 hours, and then subjected to aging at 400 ° C., A Charpy impact test was performed. As a result, in the developed material A, the decrease in the impact value change rate is longer than that of the conventional material D, and it can be said that the aged deterioration is suppressed.
[0035]
Both the developed material B and the conventional material E contain Cu, but the developed material B is subjected to a heat treatment of 1038 ° C. × 1 hour + 593 ° C. × 4 hours, and the conventional material E is 1050 ° C. × 1 hour + 580 ° C. After heat treatment for 4 hours, aging was performed at 400 ° C., and a Charpy impact test was performed. Comparing the results, it can be said that the developed material B has a lower rate of change in impact value on the longer time side than the conventional material E, and the deterioration over time is suppressed.
[0036]
Next, the developed material C, the conventional material D, and the conventional material E both have a ferrite phase. The developed material C is heat treated at 927 ° C. × 1 hour + 593 ° C. × 4 hours, and then subjected to aging at 400 ° C. Comparing the results of the impact test, it can be said that the developed material C has a lower impact value change rate for a longer time than the conventional material D and the conventional material E, and the deterioration over time is suppressed.
[0037]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, even when it uses it for a long period of time in a high temperature / high pressure water environment, the structure comprised with the stainless steel with few toughness and a ductile fall and the said stainless steel can be provided.
[Brief description of the drawings]
FIG. 1 is a table showing compositions of a developed material and a conventional material according to an embodiment of the present invention.
FIG. 2 is a graph showing changes in impact value due to 400 ° C. aging of the developed material and the conventional material shown in the table of FIG.
Claims (2)
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| JP2003109737A JP4184860B2 (en) | 2003-04-15 | 2003-04-15 | Stainless steel and structures |
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| JP2003109737A JP4184860B2 (en) | 2003-04-15 | 2003-04-15 | Stainless steel and structures |
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| JP4184860B2 true JP4184860B2 (en) | 2008-11-19 |
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| JP2006317289A (en) * | 2005-05-12 | 2006-11-24 | Toshiba Corp | Control rod drive mechanism |
| JP6583885B2 (en) * | 2015-10-20 | 2019-10-02 | 山陽特殊製鋼株式会社 | High hardness stainless steel with excellent corrosion resistance and manufacturability |
| EP3569725B1 (en) * | 2017-01-10 | 2021-03-17 | JFE Steel Corporation | Duplex stainless steel and method for producing same |
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